The invention lies in the electronics field. More specifically the invention relates to a quadricorrelator for a demodulator for frequency-modulated signals. The system is of the following type:
a first input is provided for receiving an in-phase component of a signal to be demodulated, and a second input for receiving a quadrature component of the signal to be demodulated;
the in-phase component and the quadrature component of the signal to be demodulated can be produced in a mixing stage that is connected upstream of the quadricorrelator;
the first input of the quadricorrelator is followed by a first multiplier unit, and the second input is followed by a second multiplier unit, and the output of the first multiplier unit is connected with a positive sign to a first input of an output adder unit, and the output of the second multiplier unit is connected with a negative sign to a second input of the output adder unit; and
the in-phase component which is supplied to the first input of the quadricorrelator is supplied with a first transfer function applied to it to the second multiplier unit, and the quadrature component is supplied with a second transfer function applied to it to the first multiplier unit, in each case as an input signal.
When frequency-modulation methods are used in transmission systems, such as DECT (Digital European Cordless Telecommunications standard), the signal which is modulated onto the carrier must be demodulated in the receiver chain after amplification and channel selection have been carried out. When a so-called low-IF structure (low intermediate-frequency structure) is used, the demodulation is normally carried out by means of a quadricorrelator whose demodulator characteristic is symmetrical about the frequency f=0. Such a quadricorrelator and its use in a demodulator are described, for example, in IEEE Trans. Comm. Volume COM-33, February 1985, pages 131-138. A frequency-modulated input signal is demodulated, in the structure used according to the published article, in such a way that the input signal is first of all split into two paths, with the signal in the one path being mixed with a signal in the form of a cosine wave at a specific frequency, and the signal in the other path being mixed with a signal in the form of a sine wave at the same frequency. The components of the spectrum of the mixed signal above the difference frequency are suppressed by means of a low-pass filter provided in each path. That signal path in which the input signal has been mixed with the signal in the form of a cosine wave then carries the in-phase component of the input signal, which is supplied to the first input of the actual quadricorrelator. In a corresponding way, the other signal path, in which the input signal has been mixed with the signal in the form of a sine wave, carries the quadrature component of the input signal, which is then supplied to the respective second input of the actual quadricorrelator.
The actual quadricorrelator then comprises a signal structure in which the in-phase component is differentiated in a straight-through path and is supplied to a multiplier. The quadrature component is likewise differentiated and is supplied to a further multiplier. The in-phase component is likewise supplied as an input signal to the further multiplier and the quadrature component is supplied as an input signal to the first multiplier. The output signal from the further multiplier, finally, is subtracted from the output signal from the first multiplier in order to obtain the output signal from the quadricorrelator, which corresponds to the demodulated signal.
The disadvantages of that implementation of the quadri-correlator include, firstly, the fact that the in-phase component and the quadrature component of the intermediate frequency obtained after the mixing of the input signal are not equal to zero and, thus, since the demodulator characteristic is symmetrical about the frequency f=0, the output signal has a constant offset superimposed on it. The offset is then normally removed in subsequent offset correction in order to compensate the influence of the intermediate frequency on such a demodulator characteristic during the demodulation process. This involves additional complexity.
Furthermore, the prior art configuration of a quadricorrelator has the disadvantage that, as a result of the differentiation of the in-phase component and the quadrature component in the straight-through paths through the quadricorrelator, which corresponds to multiplication of the relevant component by the transfer function of a high-pass filter in the frequency domain, the quadricorrelator has a wide noise bandwidth. With such a structure, the noise is filtered out only at very high frequencies, by virtue of the parasitic capacitances which are always present. Overall, this results in an adverse effect on the sensitivity of the quadricorrelator.
Finally, the property of the demodulator characteristic being symmetrical about the frequency f=0 results in the disadvantage that the linearity of the symmetrical demodulator characteristic must be maintained over a very wide bandwidth region about the mid-frequency f=0. This necessarily means that the demodulator characteristic must have a relatively low gradient, and thus low demodulation sensitivity as well.
The article xe2x80x9cEqualization Problems on a Digital FM Receiverxe2x80x9d by K. D. Kammeyer in Signal Processing 9(1985), pages 263-76 describes a digital receiver for frequency-modulated signals, which has digital equalizer networks and is suitable for use in FM sound radio receivers.
The article xe2x80x9cPhaselock Techniquesxe2x80x9d by F. M. Gardner, John Wiley and Sons, New York 1979, pages 84-87 describes a quadri-correlator which operates as a difference-frequency detector.
The object of the invention is to provide a quadricorrelator for a demodulator for frequency-modulated signals which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which provides for a demodulator with better sensitivity.
With the above and other objects in view there is provided, in accordance with the invention, a quadricorrelator for a demodulator for frequency-modulated signals, comprising:
a first input for receiving an in-phase component of a signal to be demodulated, and a second input for receiving a quadrature component of the signal to be demodulated;
a first multiplier unit connected to the first input and having an output, and a second multiplier unit connected to the second input and having an output, an output adder unit having a first input connected with a positive sign to the output of the first multiplier unit, a second input connected with a negative sign to the output of the second multiplier unit, and an output forming an output of the quadricorrelator;
the second multiplier unit receiving as an input signal the in-phase component from the first input via a first transfer function, and the first multiplier unit receiving as an input signal the quadrature component from the second input via a second transfer function;
the first and second transfer functions being chosen such that a demodulator characteristic formed between the input of the quadricorrelator and the output of the output adder unit cuts a frequency axis at a point where the frequency is not zero and runs essentially linearly in a sufficiently wide frequency band surrounding or adjacent to the intersection point; and
the first transfer function comprising a first transfer function element applied to the in-phase component, and a second transfer function element applied to an input signal to the first multiplier unit, wherein the signals weighted by the first transfer function element and the second transfer function element are added to generate an added signal forming the input signal to the second multiplier unit; and
the second transfer function comprising a first transfer function element applied to the quadrature component, and a second transfer function element applied to the input signal to the second multiplier unit, wherein the signals weighted by the first transfer function element and the second transfer function element are added to generate an added signal forming the input signal to the first multiplier unit.
The invention is based on the knowledge that the demodulator characteristic can be shifted in the direction of the positive or negative frequency axis by respectively providing a transmission element with a first and a second transfer function in the crossing paths of the quadricorrelator and with the first and second transfer functions being chosen appropriately.
This results in the advantage that the intersection of the demodulator characteristic with the frequency axis is closer to the (positive) intermediate frequency of the in-phase component and quadrature component, respectively, of the signal to be modulated, and the bandwidth of the demodulator characteristic within which the linearity of the characteristic must be ensured may be chosen to be narrower. Correspondingly, the gradient of the demodulator characteristic in the linear region may be chosen to be higher, thus resulting in better demodulation sensitivity.
The crossing paths of the quadricorrelator have the following structure:
The first transfer function is split into a first transfer function element which is applied to the in-phase component, and a second transfer function element which is applied to the signal supplied to the first multiplier unit, with the signals, which have been weighted by the first and the second transfer function element, being added, and the added signal being supplied to the second multiplier unit, as an input signal. In the same way, the second transfer function comprises a first transfer function element which is applied to the quadrature component, and a second transfer function element which is applied to the signal supplied to the second multiplier unit, with the signals, weighted by the first and the second transfer function element, being added, and the added signal being supplied to the first multiplier unit, as an input signal.
This structure offers the advantage of simple circuitry design in order to shift the frequency of the demodulator characteristic.
In accordance with an added feature of the invention, the intersection of the demodulator characteristic is located at an intermediate frequency of the in-phase component and the quadrature component of the signal to be demodulated. That is, the transfer functions in the crossing paths of the quadricorrelator are chosen such that the intersection of the demodulator characteristic occurs at the carrier frequency or intermediate frequency of the in-phase component and of the quadrature component of the signal to be demodulated. For the reasons mentioned above, this results in maximum sensitivity. Furthermore, the demodulated signal does not require any offset correction in this preferred embodiment of the invention. This results in reduced circuitry complexity.
In accordance with an additional feature of the invention, the first transfer function element of the first transfer function is formed by a first low-pass filterxe2x80x94preferably a single RC elementxe2x80x94and the first transfer function element of the second transfer function is formed by a second low-pass filterxe2x80x94preferably a single RC element as well.
In accordance with a preferred embodiment of the invention, the second transfer function element of the first transfer function is a first voltage-controlled current source having a voltage input receiving the signal being supplied to the first multiplier unit, and an output connected to a passive network, wherein one output voltage of the passive network represents the output signal of the second transfer function element of the first transfer function; the second transfer function element of the second transfer function is a second voltage-controlled current source having a voltage input receiving the signal being supplied to the second multiplier unit, and an output connected a passive network, wherein one output voltage of the passive network represents the output signal of the second transfer function element of the second transfer function.
In accordance with another feature of the invention, the second transfer function element of the first transfer function is equal to a negative of the second transfer function element of the second transfer function. This ensures that the demodulator characteristic has excellent linearity, without the output signal containing any higher-order components.
In accordance with a further feature of the invention, the first transfer function element of the first transfer function is equal to the first transfer function element of the second transfer function.
In accordance with a concomitant feature of the invention, the output of each of the voltage-controlled current sources is connected to the first transfer function element. That is, the outputs of each of the voltage-controlled current sources are connected to the transmission element which produces the first transfer function element.
This results in the advantage that there is no need to provide a separate adder in the crossing paths. The addition results from the fact that the current produced by the voltage-controlled current sources in each case flows through the preferably passive network of the transmission element which produces the first transfer function element. Thus, due to the superposition principle, the added voltage which is produced by the added currents appears at the output of this transmission element, with the one portion of the current being produced by the voltage-controlled current source and the other portion of the current being produced by the voltage, which is present at the input of the transmission element, of the in-phase component and quadrature component.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a quadricorrelator for a demodulator for frequency-modulated signals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.